1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a radio frequency transmitter, and more particularly, to a power amplifier circuit of a polar modulation system for envelope modulation of a high frequency signal.
2. Description of Related Art
In a transmitter of a system for high speed wireless communication such as a mobile phone, a digital multimedia broadcasting (DMB) phone, a personal digital assistant (PDA), and the like, various transmission schemes including an envelope modulation, for example, a quadrature phase shift keying (QPSK) modulation, a quadrature amplitude modulation (QAM), and the like, have been adopted. For the transmitter, a power amplifier is utilized to amplify a high frequency signal containing certain information. An output of the power amplifier is transmitted to a correspondent system via an antenna.
FIG. 1 illustrates a structure of a conventional art power amplifier 100. Referring to FIG. 1, the power amplifier 100 includes a power voltage control circuit 110 and an amplification stage 120.
An input signal PIN is a carrier signal in a high frequency domain for high speed wireless communication, and the input signal PIN is amplified by the amplification stage 120. The amplification stage 120 amplifies the input signal PIN using amplifiers in various stages. In this instance, the signal, amplified by the amplification stage 120, is mixed with an output of the power voltage control circuit 110, and the mixed signal is generated as an output signal POUT. The power voltage control circuit 110 receives and processes a predetermined signal VRAMP for specifying a voltage level of the output signal POUT, for envelope modulation of the amplified signal by the amplification stage 120.
The power voltage control circuit 110 amplifies the signal VRAMP having a predetermined envelope determined by modulation scheme, for example, QPSK modulation, QAM, and the like, or eliminates a noise component introduced into the signal VRAMP. Accordingly, the envelope modulation by the power voltage control circuit 110 is reflected to the output signal POUT, the output signal POUT is transmitted via an antenna.
A transistor is inserted to an end of the power voltage control circuit 110. Namely, the output signal POUT is generated by mixing a transistor output at the end of the power voltage control circuit 110 with the amplified signal by the amplification stage 120.
As described above, since an active device at the end of the power voltage control circuit 110 is utilized for signal amplification, a power loss becomes great and a signal distortion occurs. Namely, in FIG. 2, when the signal distortion does not exist, the output signal POUT may be output in an envelope form shaped in a waveform 210. However, when the signal distortion does exist, as shown by waveform 220, the output signal POUT is distorted in ranges where a signal level is greater than a certain level and less than a certain level due to non-linearity of the active device. Also, since the active device has a comparatively greater resistance than a passive device, power consumption is greater than the passive device. Therefore, it is required to reduce the distortion of the output signal POUT and the power loss for generating the output signal POUT.